1. Field of the Invention
The present invention relates to a light guide plate by which light being incident from a light source is let go out in a direction perpendicular to the direction of incidence, a display device, and an electronic apparatus with a display. More particularly, the invention relates to a technology by which the light guide efficiency of a light guide plate can be enhanced.
2. Description of the Related Art
Hitherto, there has been known a light guide plate which receives light incident from a light source, guides the incident light and lets the light go out in a direction perpendicular to the direction of incidence. Besides, such a guide plate has been widely used, for example, as an illuminating section (back light, etc.) in a liquid crystal display device.
FIG. 14 is a side view of a liquid crystal display device 101 using a light guide plate 121 according to related art.
As shown in FIG. 14, the liquid crystal display device 101 includes a light source 191, an LCD (liquid crystal display) 111 in which the contents being displayed on the front side (the upper side in FIG. 14) thereof is made clearly visible by irradiation with the light of the light source 191, and a light guide plate 121 by which the light from the light source 191 is guided to the back side (the lower side in FIG. 14) of the LCD 111. Incidentally, a polarizing plate 113 is disposed on the front side of the LCD 111, and a polarizing plate 114 is disposed on the back side of the LCD 111. In addition, a lens sheet 116 and a diffuser 118 are sequentially arranged between the polarizing plate 114 and the light guide plate 121. Further, a back reflector 194 is disposed on the back side of the light guide plate 121.
In the liquid crystal display device 101 as above, the light source 191 is disposed to face a side end face 131a of an incoming part 131 of the light guide plate 121. Therefore, upon incidence on the side end face 131a, the light from the light source 191 is guided through the incoming part 131 into a main body part 132, and is let go out from a flat principal surface 132a toward the back surface of the LCD 111. In other words, the light guide plate 121 constitutes a back light for the LCD 111.
Here, for making the liquid crystal display device 101 smaller in size, the light guide plate 121 is reduced in thickness at the main body part 132 located on the back side of the LCD 111. On the other hand, for securing a sufficient amount of light, the thickness of the incoming part 131 is set large on the side of the side end face 131a, in conformity with the light source 191 which is comparatively large in size. Therefore, the front surface of the incoming part 134 has an inclined front surface 133a which is inclined from the side end face 131a toward the principal surface 132a. In addition, the back surfaces of the incoming part 131 and the main body part 132 constitute a flat back surface 133b. 
Thus, in the light guide plate 121 according to the related art, the thickness of the incoming part 131 on the side of the side end face 131a is greater than the thickness of the main body part 132, and both the parts are connected to each other through the inclined front surface 133a and the flat back surface 133b. Therefore, there has been a problem that light leaks through the inclined front surface 133a, whereby the light guide efficiency of the light guide plate 121 is lowered.
FIGS. 15A and 15B illustrate the conditions of generation of light leakage due to the light guide plate 121 in the related art.
Incidentally, FIG. 15A schematically illustrates the path of a ray of light coming from the light source 191 and entering the light guide plate 121, whereas FIG. 15B illustrates simulational determination of various paths of rays of light from the light source 191.
As shown in FIG. 15A, when a ray of light coming from the light source 191 and being incident on the incoming part 131 of the light guide plate 121 (a ray of light parallel to the principal surface 132a of the main body part 132) passes through the side end face 131a and impinges on the inclined front surface 133a (set at angle θ against the principal surface 132a), the ray of light is reflected (at reflection angle θ) on the inclined front surface 133a, and is then reflected (at reflection angle 2θ) on the flat back surface 133b. Thereafter, the ray of light impinges again on the inclined front surface 133a; in this instance, if the angle of impingement on the inclined front surface 133a is greater than a critical angle for total reflection, the light would leak through the inclined front surface 133a. 
FIG. 15B shows the simulationally determined conditions of light leakage. As is seen from the figure, if the incoming part 131 is configured to have the inclined front surface 133a and the flat back surface 133b, much light leaks through the inclined front surface 133a. According to the simulation results, it is seen that when the plate thickness at the side end face 131a is 0.5 mm and the thickness of the main body part 132 is 0.25 mm, 26% of the incident light is lost through the inclined front surface 133a before reaching the principal surface 132a, and the total light guide efficiency determined taking other losses also into account is as low as 64%. Incidentally, on the side of the flat back surface 133b, the back reflector 194 for reflecting the leaking light is arranged, so that light leakage does not occur on this side.
In view of the foregoing, the present applicant has already proposed a technology by which the light guide efficiency of a light guide plate can be enhanced. In the technology, an incoming part, at a side end face opposed to a light source, of a light guide plate is split into a plurality of portions stacked in the thickness direction, these split portions are mutually staggered sideways along a direction parallel to a principal surface of the light guide plate by lateral light guide parts and flat light guide parts, and light is bent in the thickness direction and guided to a main body part of the light guide plate by a bent light guide part of which plain surfaces on opposite sides are substantially parallel to each other (refer to, for example, Japanese Patent Laid-Open No. 2006-351511, hereinafter referred to as Patent Document 1).